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Why doesn't it turn

I have just finished a rebuild on a York LTD-126 compressor, not my first one. This time I have a problem. The input shaft will not turn if the system is in a vacuum, at atmospheric pressure it seems tight to start movement, but as it turns it gets easier. When under nitrogen pressure above 3-4#s it moves freely. All the tolerances are to the York manual for thrust (low and high speed) run out on the impeller is .0005" and the eye is .0000. I made sure it kept turning free during assembly. HELP! please. -GEO

Once in a while everything falls into place and I am able to move forward, most of the time it just falls all over the place and I can't go anywhere-GEO

Never had this before

I don't know exactly what is causing it, but it is strange. I have rebuilt many York's including several large LTD's and this has never happened. The pressure issue leads me to believe something with the new seal as that's the only place I can think of that would have a pressure differential. When I went to pressure check my work I found the seal not holding. I went back in the seal cavity and found the faces were dry, they were coated with clean "C" at assembly. When I went to try a vacuum pull I could not get below 26"hg, same problem. With system in a vacuum you can't turn the shaft even with a persuader. If I put a 5# charge of nitrogen it will spin with your hand. At 0 psig you need a wrench to start it, but it turns.-GEO

Once in a while everything falls into place and I am able to move forward, most of the time it just falls all over the place and I can't go anywhere-GEO

The reason I said that it sounds like a low speed thrust issue is that when you pressurize the machine it forces the low speed shaft out and towards the counterthrust position (loads the seal face pressure as it does this). When you put the machine under vacuum it tries to pull the low speed shaft in towards the thrust position (and unloads the seal faces). It's really anybodies guess as to the exact problem, but it does sound like you may have to re-enter the machine to find out. Maybe an anti-rotation pin broke off and it's allowing a bearing to cock sideways in the housing when the shaft pulls in? I thought about the chances of the high speed gear riding the low speed when it pulled in and forcing the impeller against the eye seal, but unless something was installed wrong (in which case your measurements would have been off terribly), I don't see how that could happen. Be interested to know what you find.

I hope not

klove, I appreciate your reasoning and it does make sense when you think about it. Anytime I am dealing with anti-rotation pins I get extremely careful on marking the location of the pin and locating hole were it rest. I mark the exact placement and put grease in front and behind it, but not on it. Then I install the piece completely and remove it, if the pin is clean (no grease) it's in correctly. That along with making sure the edges of the bearing are all the way to the land on the rotor casing. The oil pump (low speed) ring also has the large anti-rotation machined hole that mate to another larger pin in the casing of the rotor housing that holds the low speed pump outlet into the oil way machined into the casing. I hope your not correct this time as I really don't like the idea of pulling that rotor section back down out of the compressor. The LTD-126 is a lot more difficult then any in the series because of that cross member 6" behind the bottom of the scroll/rotor support. You kind of have to balance it at an angle to get it out without hitting the impeller. I will keep you updated as I go along. As of now the powers that be wanted a full charge of oil put in and the heaters energized, thinking that will keep it free.-GEO

Once in a while everything falls into place and I am able to move forward, most of the time it just falls all over the place and I can't go anywhere-GEO